1. Field of the Invention
The present invention relates to a system for controlling an idle running condition of an engine and particularly to a system for controlling an idling speed of a vehicular engine.
2. Description of the Prior Art
Heretofore, there has been proposed a vehicular engine idling speed controlling system in which engine speed, throttle valve opening, vehicle speed, etc. are detected respectively by engine speed sensor, throttle position sensor, vehicle speed sensor, etc. and a control signal based on those detected signals is fed to an actuator (motor) of a stopper member (rod) which restricts a stop position on a closing side of a throttle valve, to move the rod forward or backward to shift the closing side stop position of the throttle valve, thereby effecting an engine speed feedback control (NFB control) under a certain condition I during idle running, while permitting a throttle valve position feedback (PFB) control under another condition II during idle running.
The above condition I indicates the case where at least the following requirements are satisfied, in which the engine is in a relatively stable state:
(1) A predetermined time should have been elapsed after changing from OFF to ON of an idle switch.
(2) The vehicle speed be extremely low (e.g. not higher than 2.0 km/h); in other words, a signal frequency provided from a vehicle speed sensor which detects a vehicle speed using a pulse signal having a frequency porportional to a vehicle speed, should be not higher than a predetermined value.
(3) In a vehicle having a cooler, a predetermined time should have been elapsed after switching over of a cooler relay or the like according to a cooler load.
(4) In a vehicle having an automatic transmission with turbo type fluid coupling, a predetermined time should have been elapsed after switch-over of speed change range from neutral range (including parking range) to running range or vice versa.
The above condition II indicates the case where the above condition I is not satisied, the engine is not relatively stable, and a rapid feedback control is desired.
In conventional engine idling speed controlling systems, when the idling speed control is made according to PFB control under the above condition II, the throttle opening is determined in accordance with a detected signal (actual throttle opening signal) provided from a throttle sensor, but there exists the problem that the relation between actual throttle opening and engine idling speed changes due to a setting error of the throttle sensor or a change with time of friction of a driving mechanism for a throttle valve.
In an attempt to solve the above problem there has been proposed a learning means which stores the relation between throttle valve position and engine speed during NFB control and makes a correction control using such stored value during PFB control. However, the following problems are involved therein.
(1) If the relation between throttle opening and engine speed is corrected always with the newest data, then in the event the load in NFB control changes in PFB control, particularly in the event of a lighter load in PFB control, the engine speed rises and the feeling becomes worse. For example, there arises an engine speed rise of about 400 rpm at a power steering load, and also at an electric load there arises an engine speed rise not less than 200 rpm.
(2) If the relation between throttle opening and engine speed is corrected with a mean value over a long time, the engine speed varies when an electric load turns ON and the load increases under conditions under which NFB control and PFB control are repeated frequently.